Micro-electro-mechanical (MEM) dies, such as gyroscopes and accelerometers are typically very small and sensitive to thermal and mechanical stresses. In the prior art, the sensor die is rigidly mounted to the floor or substrate of a ceramic chip carrier package to maintain axis alignment, bias stability, and scale factor stability with respect to the package over time and temperature variations. The mounting scheme used strives to maintain rigidity and stability over a wide range of environmental conditions, such as temperature cycling, vibrational stresses, and g-loading. At the same time, the package should not impart any significant stresses on the sensor over these same environmental conditions. Furthermore, any minor stresses that do occur should be stable and cannot exhibit any hysteresis effects which would affect performance of the sensor.
As stated above, the MEM die, such as a gyroscope or accelerometer sensor, is typically directly bonded to the package, or chip carrier. Aluminum oxide is often used as the material for a fabricating a hermetic chip carrier for such MEM devices. In some cases, a small mounting pad may be placed between the floor of the chip carrier and the sensor in an attempt to absorb various stresses and strains which occur during the operation of the device. Another prior art technique to reduce thermal stress on the sensor is to braze the sensor directly to a package made of a material more closely matching the thermal expansion coefficient of the sensor, such as aluminum nitride.
These prior art techniques exhibit several distinct disadvantages. When the sensor is directly bonded to the package, a large thermal stress develops because the thermal expansion coefficient of the package often greatly exceeds the thermal expansion coefficient of the sensor. Furthermore, the braze materials may impart similar thermal stress as stated above. The result is warping of the sensor which adversely affects its performance. This prior art technique also makes the sensor susceptible to any externally applied forces.
Prior art techniques which attempt to reduce thermal and mechanical stresses by utilizing a small pad near the center of the sensor require precise control of the quantity of solder used, as well as precise control of the placement of the die. Using this prior art technique, localized attachment is susceptible to alignment shifts due to stress relaxation of the brazed connection.
If expensive aluminum nitride is used as the package material because this material has a thermal expansion coefficient which more closely matches the thermal coefficient of the sensor, the result is a package which can cost as much as eight times more than an aluminum oxide package.
Additionally, brazing the die to the floor of the package still leaves it susceptible to strains and stresses due to the flexing of the package or chip carrier floor. Finally, the prior art techniques do not always adequately control thermal stresses, are not easily scalable to large size dies, are not mechanically robust, and can be expensive to implement because they require specialized materials and/or assembly methods.